1. Field of the Invention
This invention relates to a new process for the preparation of polyisocyanates containing isocyanurate groups by the partial trimerization of the isocyanate groups of organic polyisocyanates or of mixtures of di- and mono-isocyanates, a catalyst solution suitable for carrying out this process, and the use of the products of the process as an isocyanate component in the production of polyurethanes by the isocyanate-polyaddition process.
2. Description of the Prior Art
Processes for the trimerization of organic isocyanates, in particular of polyisocyanates, are known in large numbers, (J. H. Saunders and K. C. Frisch, Polyurethanes Chemistry and Technology, page 94 et seq (1962)). Strong organic bases are suitable catalysts for this trimerization, e.g. those metal salts of carboxylic acids which are alkaline in reaction, metal alcoholates, metal phenolates, alkali metal carbonates, tert. amines, tert. phosphines and the "onium" compounds of nitrogen and phosphorus, and basic heterocyclic compounds of these elements. The catalysts are frequently used as combinations or together with other cocatalysts such as mono-N-substituted carbamic acid esters (A. Farkas and G. A. Mills, Advances in Catalysis, Vol. 13, 393 (1962)).
Most processes for the preparation of high quality polyisocyanates containing isocyanurate groups use expensive catalyst systems since it is known that simple metal salts such as carboxylates or alcohols are only capable of effecting cyclotrimerization of isocyanates at relatively high concentrations and at a high temperature; the trimerization of 5 parts of phenylisocyanate, for example, requires 1 part of potassium acetate and heating for 3 hours at 100.degree. C. (Houben-Weyl, Vol. 8, page 244, publishers Thieme Verlag) (see also British Pat. No. 809,809, Example 6).
If trimerization with metal salts is to be carried out in solvents, highly polar aprotic solvents such as dimethyl formamide or dimethylsulphoxide must be used because only these are capable of dissolving inorganic metal salts and metal salts with a small organic group (German Offenlegungsschrift No. 2,839,084). Even then, catalyst concentrations of as much as 0.1 to 0.5% by weight are required. This also applies when the solvents used are protic but form urethanes with the isocyanate, thereby lowering the isocyanate content, or form precipitates and cloudiness so that the reaction product must be filtered (British Pat. No. 920,080).
Furthermore, the metal salts known in the art effect rapid trimerization only in the case of aromatic isocyanates while aliphatic mono- and polyisocyanates require a high catalyst concentration and temperatures above 50.degree. C., whereby the reaction frequently takes an uneven exothermic course and in the case of polyisocyanates results in highly viscous, strongly discolored products (see U.S. Pat. No. 3,330,828, Examples 1 to 4; British Pat. No. 952,931, Example 3; German Auslegeschrift No. 1,013,869, Example 3) or the formation of gel particles (British Pat. No. 966,338, Example 3), whereby the products become to a large extent unsuitable for use in polyurethane lacquers. Another major disadvantage of using metal salts as catalysts is that stopping the catalyst results in the formation of inorganic salts which are insoluble in the polyisocyanate and cause cloudiness. The more recent processes of the art therefore rarely use the simple and inexpensive basic metal salts such as potassium acetate but special organic bases, depending on the particular isocyanate, and these are used under quite specific reaction conditions. Thus, for example, the trimerization of aromatic polyisocyanates is carried out using Mannich bases (German Offenlegungsschrift No. 2,551,634 and German Offenlegungsschrift No. 2,641,380) or tertiary phosphines, in which case uretdiones are first formed which are converted to the isocyanurate only in a second phase of the reaction (German Offenlegungsschrift No. 1,201,992). Organic bases having a betaine structure, such as quaternary ammonium hydroxides (European Offenlegungsschrift No. 010,589 and European Offenlegungsschrift No. 009,694), aminimides (J. E. Kresta, R. J. Chang, S. Kathiriya and K. C. Frisch, Makromol. Chem. 180, 1081 (1979)) and azirine derivatives in combination with tert. amines (German Auslegeschrift No. 2,325,826) are frequently used for the trimerization of (cyclo)aliphatic diisocyanates.
One disadvantage of all these catalyst systems is that quite specific temperature intervals must be observed and the process may in part only be carried out solvent-free and in part only in selected solvents, and in particular trimerization can only be carried out on aromatic polyisocyanates alone or aliphatic polyisocyanates alone.
It is an object of the present invention to provide a process by which colorless aromatic and aliphatic polyisocyanates containing isocyanurate groups may be obtained by a technically simple procedure either in solvents or solvent-free and without elaborate temperature control, using one and the same catalyst.
It was surprisingly found that this problem could be solved by carrying out the trimerization using 1:1-complexes of basic sodium or potassium compounds with 1,4,7,10,13-pentaoxacyclopentadecane ("15-crown-5") or 1,4,7,10,13,16-hexaoxacyclooctadecane ("18-crown-6") as trimerization catalysts.
Although C. J. Pedersen already recognized that crown-ether-complexed alkali metal salts are fundamentally suitable as trimerization catalysts for aromatic isocyanates (J. Am. Chem. Soc. 89, 7017 (1967) or U.S. Pat. No. 3,686,225), the crown ethers containing condensed benzene or cyclohexane rings described in these prior publications and their complexes with basic sodium or potassium compounds are not suitable for large scale technical production of high quality polyisocyanates containing isocyanurate groups, firstly because the crown ethers with condensed cyclohexane rings have only an extremely slight complex forming action on basic alkali metal compounds and, secondly, because 1:1 complexes based on crown ethers having condensed benzene rings have very poor solubility in organic media so that, for example, concentrated catalyst solutions in physiologically harmless solvents cannot be prepared using these complexes. By contrast, the 1:1 complexes described below which are to be used in the process according to the invention do not have such disadvantages. The crown ethers underlying the 1:1 complexes which are an essential feature of this invention are eminently suitable for the preparation of stable complexes with basic sodium and potassium compounds and at the same time these complexes are readily soluble both in the polyisocyanates to be trimerized and in the auxiliary solvents which will be described below. They may therefore be added as relatively concentrated solutions to the trimerizing polyisocyanate, a factor which is important for the large scale technical production of polyisocyanates having isocyanurate groups.